Decoding method, memory storage device and memory control circuit unit

ABSTRACT

A decoding method, a memory storage device and a memory control circuit unit are provided, the decoding method includes: reading a plurality of memory cells according to hard decision voltage to obtain hard bit; performing a parity check procedure for the hard bit to obtain a plurality of syndromes; determining whether the hard bit has error according to the syndromes; if the hard bit has the error, updating the hard bit according to channel information of the hard bit and syndrome weight information corresponding to the hard bit.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan applicationserial no. 103113690, filed on Apr. 15, 2014. The entirety of theabove-mentioned patent application is hereby incorporated by referenceherein and made a part of this specification.

BACKGROUND

1. Technical Field

The invention relates to a decoding method, and more particularly, to adecoding method regarding a low density parity check code, a memorystorage device and a memory control circuit unit using the same.

2. Description of Related Art

The markets of digital cameras, cellular phones, and MP3 players haveexpanded rapidly in recent years, resulting in escalated demand forstorage media by consumers. The characteristics of data non-volatility,low power consumption, and compact size make a rewritable non-volatilememory module (e.g., flash memory) ideal to be built in the portablemulti-media devices as cited above.

Generally, a channel encoding is performed to data written in therewritable non-volatile memory module. As a result, when the data areread from the rewritable non-volatile memory module, errors in the datamay have a chance to be corrected. In case the channel encoding utilizesa low density parity check code, an iteration decoding is performed tothe data read from the rewritable non-volatile memory module. Theiteration decoding is configured to update a reliability of one bit. Anumber of iterations required for the iteration decoding also increaseswhen there is more errors in the data. However, a higher number ofiterations results in a slower speed for decoding. Therefore, how toincrease a speed of decoding is one of the major subjects for personskilled in the art.

Nothing herein should be construed as an admission of knowledge in theprior art of any portion of the present invention. Furthermore, citationor identification of any document in this application is not anadmission that such document is available as prior art to the presentinvention, or that any reference forms a part of the common generalknowledge in the art.

SUMMARY

The invention is directed to a decoding method, a memory storage deviceand a memory control circuit unit, and capable of collecting additionalchannel information to serve as a basis of correcting error.

A decoding method is provided according an exemplary embodiment of theinvention for a rewritable non-volatile memory module having a pluralityof memory cells, and the decoding method includes: reading the memorycells according to at least one hard decision voltage to obtain at leastone hard bit; performing a parity checking procedure for the hard bit toobtain a plurality of syndromes, wherein the hard bit is correspondingto at least one of the syndromes; determining whether the hard bit hasat least one error according to the syndromes; if the hard bit has theerror, updating the hard bit according to channel information of thehard bit and syndrome weight information corresponding to the hard bit;and if the hard bit does not have the error, outputting the hard bit.

A memory storage device is provided according to an exemplary embodimentof the invention, which includes a connection interface unit, arewritable non-volatile memory module and a memory control circuit unit.The connection interface unit is configured to couple to a host system.The rewritable non-volatile memory module includes a plurality of memorycells. The memory control circuit unit is coupled to the connectioninterface unit and the rewritable non-volatile memory module. Therein,the memory control circuit unit is configured to read the memory cellsaccording to at least one hard decision voltage to obtain at least onehard bit. The memory control circuit unit is further configured toperform a parity checking procedure for the hard bit to obtain aplurality of syndromes, wherein the hard bit is corresponding to atleast one of the syndromes. The memory control circuit unit is furtherconfigured to determine whether the hard bit has at least one erroraccording to the syndromes. If the hard bit has the error, the memorycontrol circuit unit is further configured to update the hard bitaccording to channel information of the hard bit and syndrome weightinformation corresponding to the hard bit. If the hard bit does not havethe error, the memory control circuit unit is further configured tooutput the hard bit.

A memory control circuit unit for a rewritable non-volatile memorymodule is provided, and the rewritable non-volatile memory moduleincludes a plurality of memory cells, and the memory control circuitunit includes a host interface, a memory interface, an error checkingand correcting circuit, and a memory management circuit. The hostinterface is configured to couple to a host system. The memory interfaceis used for coupling to the rewritable non-volatile memory module. Thememory management circuit is coupled to the host interface, the memoryinterface and the error checking and correcting circuit. Therein, thememory control circuit unit is configured to send a read commandsequence, wherein the read command sequence is configured to instructfor reading the memory cells according to at least one hard decisionvoltage to obtain at least one hard bit. The error checking andcorrecting circuit is configured to perform a parity checking procedurefor the hard bit to obtain a plurality of syndromes, wherein the hardbit is corresponding to at least one of the syndromes. The errorchecking and correcting circuit is further configured to determinewhether the hard bit has at least one error according to the syndromes.If the hard bit has the error, the error checking and correcting circuitis further configured to update the hard bit according to channelinformation of the hard bit and syndrome weight informationcorresponding to the hard bit. If the hard bit does not have the error,the memory management circuit is further configured to output the hardbit.

Based on above, when the bit read from the rewritable non-volatilememory module includes the error, an exemplary embodiment of theinvention is capable of deciding which bits are to be updated accordingto the channel information of each of the bits and the syndrome weightinformation corresponding to each of the bits. Accordingly, incomparison to conventional method in which the codeword is updated onlyaccording to a result of the iteration calculation each time, anexemplary embodiment of the invention is capable of effectivelyimproving decoding efficiency.

To make the above features and advantages of the disclosure morecomprehensible, several embodiments accompanied with drawings aredescribed in detail as follows.

It should be understood, however, that this Summary may not contain allof the aspects and embodiments of the present invention, is not meant tobe limiting or restrictive in any manner, and that the invention asdisclosed herein is and will be understood by those of ordinary skill inthe art to encompass obvious improvements and modifications thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a schematic diagram of a host system and a memory storagedevice according to an exemplary embodiment of the invention.

FIG. 2 is a schematic diagram illustrating a computer, an input/outputdevice and a memory storage device according to an exemplary embodiment.

FIG. 3 is a schematic diagram of a host system and a memory storagedevice according to an exemplary embodiment.

FIG. 4 is a schematic block diagram of the memory storage device in FIG.1.

FIG. 5 is a schematic block diagram illustrating a memory controlcircuit unit according to an exemplary embodiment.

FIG. 6 is a schematic diagram illustrating a parity check matrixaccording to an exemplary embodiment.

FIG. 7 is a schematic diagram illustrating an example of reading therewritable non-volatile memory module according to a hard decisionvoltage according to an exemplary embodiment.

FIG. 8 is a schematic diagram illustrating the hard bit and the soft bitcorresponding to the distributions of threshold voltages of the SLC-typeflash memory module according to an exemplary embodiment.

FIG. 9 is a schematic diagram illustrating distributions of thresholdvoltages of the MLC-type flash memory module according to an exemplaryembodiment.

FIG. 10 and FIG. 11 are schematic diagrams illustrating that the hardbit and the soft bit corresponding to the distributions of thresholdvoltages of the MLC-type flash memory module according to an exemplaryembodiment.

FIG. 12 to FIG. 14 are schematic diagrams illustrating that the hard bitand the soft bit corresponding to the distributions of thresholdvoltages of the TLC-type flash memory module according to an exemplaryembodiment.

FIG. 15 is a schematic diagram illustrating a matrix multiplicationaccording to an exemplary embodiment.

FIG. 16 is a schematic diagram illustrating that the hard bit and thesoft bit corresponding to the distributions of threshold voltages of theSLC-type flash memory module according to an exemplary embodiment.

FIG. 17 is a flowchart illustrating a decoding method according to anexemplary embodiment.

FIG. 18 is a flowchart illustrating a decoding method according toanother exemplary embodiment.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

Embodiments of the present invention may comprise any one or more of thenovel features described herein, including in the Detailed Description,and/or shown in the drawings. As used herein, “at least one”, “one ormore”, and “and/or” are open-ended expressions that are both conjunctiveand disjunctive in operation. For example, each of the expressions “atleast on of A, B and C”, “at least one of A, B, or C”, “one or more ofA, B, and C”, “one or more of A, B, or C” and “A, B, and/or C” means Aalone, B alone, C alone, A and B together, A and C together, B and Ctogether, or A, B and C together.

It is to be noted that the term “a” or “an” entity refers to one or moreof that entity. As such, the terms “a” (or “an”), “one or more” and “atleast one” can be used interchangeably herein.

Generally, a memory storage device (also known as a memory storagesystem) includes a rewritable non-volatile memory module and acontroller (also known as a control circuit). The memory storage deviceis usually configured together with a host system so that the hostsystem may write data into or read data from the memory storage device.

FIG. 1 is a schematic diagram of a host system and a memory storagedevice according to an exemplary embodiment. FIG. 2 is a schematicdiagram illustrating a computer, an input/output device and a memorystorage device according to an exemplary embodiment.

Referring to FIG. 1, a host system 1000 includes a computer 1100 and aninput/output (I/O) device 13. The computer 1100 includes amicroprocessor 11, a random access memory (RAM) 12, a system bus 14, anda data transmission interface 15. For example, the I/O device 13includes a mouse 16, a keyboard 17, a display 18 and a printer 19 asshown in FIG. 2. It should be understood that the devices illustrated inFIG. 2 are not intended to limit the I/O device 13, and the I/O device13 may further include other devices.

In the present embodiment of the invention, the memory storage device100 is coupled to other devices of the host system 1000 through the datatransmission interface 15. By using the microprocessor 11, the randomaccess memory 12 and the Input/Output (I/O) device 13, data may bewritten into the memory storage device 100 or may be read from thememory storage device 100. For example, the memory storage device 100may be a rewritable non-volatile memory storage device such as a flashdrive 20, a memory card 21, or a solid state drive (SSD) 22 as shown inFIG. 2.

FIG. 3 is a schematic diagram of a host system and a memory storagedevice according to an exemplary embodiment.

Generally, the host system 1000 may substantially be any system capableof storing data with the memory storage device 100. Although the hostsystem 1000 is described as a computer system in the present exemplaryembodiment, in another exemplary embodiment of the invention, the hostsystem 1000 may be a digital camera, a video camera, a telecommunicationdevice, an audio player, or a video player. For example, when the hostsystem is a digital camera (video camera) 23, the rewritablenon-volatile memory storage device may be a SD card 24, a MMC card 25, amemory stick 26, a CF card 27 or an embedded storage device 28 (as shownin FIG. 3). The embedded storage device 28 includes an embedded MMC(eMMC). It should be mentioned that the eMMC is directly coupled to asubstrate of the host system.

FIG. 4 is a schematic block diagram of the memory storage device in FIG.1.

Referring to FIG. 4, the memory storage device 100 includes a connectioninterface unit 102, a memory control circuit unit 104 and a rewritablenon-volatile memory storage module 106.

In the present exemplary embodiment, the connection interface unit 102is compatible with a serial advanced technology attachment (SATA)standard. However, the invention is not limited thereto, and theconnection interface unit 102 may also be compatible with a ParallelAdvanced Technology Attachment (PATA) standard, an Institute ofElectrical and Electronic Engineers (IEEE) 1394 standard, a peripheralcomponent interconnect (PCI) Express interface standard, a universalserial bus (USB) standard, a secure digital (SD) interface standard, aUltra High Speed-I (UHS-I) interface standard, a Ultra High Speed-II(UHS-II) interface standard, a memory stick (MS) interface standard, amulti media card (MMC) interface standard, an embedded MMC (eMMC)interface standard, a Universal Flash Storage (UFS) interface standard,a compact flash (CF) interface standard, an integrated deviceelectronics (IDE) interface standard or other suitable standards. Theconnection interface unit 102 and the memory control circuit unit 104may be packaged into one chip, or the connection interface unit 102 isdistributed outside of a chip containing the memory control circuit unit104.

The memory control circuit unit 104 is configured to execute a pluralityof logic gates or control commands which are implemented in a hardwareform or in a firmware form, so as to perform operations of writing,reading or erasing data in the rewritable non-volatile memory storagemodule 106 according to the commands of the host system 1000.

The rewritable non-volatile memory storage module 106 is coupled to thememory control circuit unit 104 and configured to store data writtenfrom the host system 1000. The rewritable non-volatile memory storagemodule 106 has multiple physical erasing units 304(0) to 304(R). Forexample, the physical erasing units 304(0) to 304(R) may belong to thesame memory die or belong to different memory dies. Each physicalerasing unit has a plurality of physical programming units, and thephysical programming units of the same physical erasing unit may bewritten separately and erased simultaneously. For example, each physicalerasing unit is composed by 128 physical programming units.Nevertheless, it should be understood that the invention is not limitedthereto. Each physical erasing unit is composed by 64 physicalprogramming units, 256 physical programming units or any amount of thephysical programming units.

More specifically, each of the physical programming units includes aplurality of word lines and a plurality of bit lines, and a memory cellis disposed at an intersection of each of the word lines and each of thedata lines. Each memory cell can store one or more bits. All of thememory cells in the same physical erasing unit are erased together. Inthe present exemplary embodiment, the physical erasing unit is a minimumunit for erasing. Namely, each physical erasing unit contains the leastnumber of memory cells to be erased together. For instance, the physicalerasing unit is a physical block. Furthermore, the memory cells on thesame word line can be grouped into one or more of the physicalprogramming units. In case each of the memory cells can store more thantwo bits, the physical programming units on the same word line can beclassified into a lower physical programming unit and an upper physicalprogramming unit. Generally, a writing speed of the lower physicalprogramming unit is faster than a writing speed of the upper physicalprogramming unit. In the present exemplary embodiment, the physicalprogramming unit is a minimum unit for programming. That is, thephysical programming unit is the minimum unit for writing data. Forexample, the physical programming unit is a physical page or a physicalsector. In case the physical programming unit is the physical page, eachphysical programming unit usually includes a data bit area and aredundancy bit area. The data bit area has multiple physical sectorsconfigured to store user data, and the redundant bit area is configuredto store system data (e.g., an error correcting code). In the presentexemplary embodiment, each of the data bit areas contains 32 physicalsectors, and a size of each physical sector is 512-byte (B). However, inother exemplary embodiments, the data bit area may also include 8, 16,or more or less of the physical sectors, and amount and sizes of thephysical sectors are not limited in the invention.

In the present exemplary embodiment, the rewritable non-volatile memorymodule 106 is a Single Level Cell (SLC) NAND flash memory module inwhich one memory cell is capable of storing one bit. However, theinvention is not limited thereto. The rewritable non-volatile memorymodule 106 may also be a Multi Level Cell (MLC) NAND flash memorymodule, a Trinary Level Cell (TLC) NAND flash memory module, other flashmemory modules or any memory module having the same features.

FIG. 5 is a schematic block diagram illustrating a memory controlcircuit unit according to an exemplary embodiment.

Referring to FIG. 5, the memory control circuit unit 104 includes amemory management circuit 202, a host interface 204 and a memoryinterface 206.

The memory management circuit 202 is configured to control overalloperations of the memory control circuit unit 104. Specifically, thememory management circuit 202 has a plurality of control commands.During operations of the memory storage device 100, the control commandsare executed to perform various operations such as writing, reading anderasing data. Operations of the memory management circuit 202 aresimilar to the operations of the memory control circuit unit 104, thusrelated description is omitted hereinafter.

In the present exemplary embodiment, the control commands of the memorymanagement circuit 202 are implemented in a faun of a firmware. Forinstance, the memory management circuit 202 has a microprocessor unit(not illustrated) and a ROM (not illustrated), and the control commandsare burned into the ROM. When the memory storage device 100 operates,the control commands are executed by the microprocessor to performoperations of writing, reading or erasing data.

In another exemplary embodiment of the invention, the control commandsof the memory management circuit 202 may also be stored as program codesin a specific area (for example, the system area in a memory exclusivelyused for storing system data) of the rewritable non-volatile memorymodule 106. In addition, the memory management circuit 202 has amicroprocessor unit (not illustrated), a ROM (not illustrated) and a RAM(not illustrated). More particularly, the ROM has a boot code, which isexecuted by the microprocessor unit to load the control commands storedin the rewritable non-volatile memory module 106 to the RAM of thememory management circuit 202 when the memory control circuit unit 104is enabled. Next, the control commands are executed by themicroprocessor unit to perform operations of writing, reading or erasingdata.

Further, in another exemplary embodiment of the invention, the controlcommands of the memory management circuit 202 may also be implemented ina form of hardware. For example, the memory management circuit 202includes a microcontroller, a memory writing unit, a memory readingunit, a memory erasing unit and a data processing unit. The memorymanagement unit, the memory writing unit, the memory reading unit, thememory erasing unit and the data processing unit are coupled to themicroprocessor. The memory management unit is configured to manage thephysical erasing units of the rewritable non-volatile memory module 106;the memory writing unit is configured to issue a write command to therewritable non-volatile memory module 106 in order to write data to therewritable non-volatile memory module; the memory reading unit isconfigured to issue a read command to the rewritable non-volatile memorymodule 106 in order to read data from the rewritable non-volatile memorymodule 106; the memory erasing unit is configured to issue an erasecommand to the rewritable non-volatile memory module 106 in order toerase data from the rewritable non-volatile memory module 106; the dataprocessing unit is configured to process both the data to be written tothe rewritable non-volatile memory module 106 and the data to be readfrom the rewritable non-volatile memory module 106.

The host interface 204 is coupled to the memory management circuit 202and configured to receive and identify commands and data sent from thehost system 1000. Namely, the commands and data sent from the hostsystem 1000 are passed to the memory management circuit 202 through thehost interface 204. In the present exemplary embodiment, the hostinterface 204 is compatible with a SATA standard. However, it should beunderstood that the present invention is not limited thereto, and thehost interface 204 may also be compatible with a PATA standard, an IEEE1394 standard, a PCI Express standard, a USB standard, a SD standard, aUHS-I standard, a UHS-II standard, a MS standard, a MMC standard, a eMMCstandard, a UFS standard, a CF standard, an IDE standard, or othersuitable standards for data transmission.

The memory interface 206 is coupled to the memory management circuit 202and configured to access the rewritable non-volatile memory module 106.That is, data to be written to the rewritable non-volatile memory module106 is converted to a format acceptable to the rewritable non-volatilememory module 106 through the memory interface 206.

In an exemplary embodiment of the invention, the memory control circuitunit 104 further includes a buffer memory 252, a power managementcircuit 254 and an error checking and correcting circuit 256.

The buffer memory 252 is coupled to the memory management circuit 202and configured to temporarily store data and commands from the hostsystem 1000 or data from the rewritable non-volatile memory module 106.

The power management unit 254 is coupled to the memory managementcircuit 202 and configured to control a power of the memory storagedevice 100.

The error checking and correcting circuit 256 is coupled to the memorymanagement circuit 202 and configured to perform an error checking andcorrecting process to ensure the correctness of data. Specifically, whenthe memory management circuit 202 receives a write command from the hostsystem 1000, the error checking and correcting circuit 256 generates anerror correcting code (ECC code) for data corresponding to the writecommand, and the memory management circuit 202 writes data and the ECCcode corresponding to the write command to the rewritable non-volatilememory module 106. Subsequently, when the memory management circuit 202reads the data from the rewritable non-volatile memory module 106, theECC code corresponding to the data is also read, and the error checkingand correcting circuit 256 may execute the error checking and correctingprocedure for the read data according to the ECC code.

In the present exemplary embodiment, the error correcting code used bythe error checking and correcting circuit 256 is a low density paritycheck (LDPC) code. In other words, the error checking and correctingcircuit 256 encodes and decodes according to a low density parity checkalgorithm. The error checking and correcting circuit 256 sets a paritycheck matrix with a dimension being “m-by-n”. Therein, m and n arepositive integers. The positive integers n indicate a number of bits inone codeword, and the positive integer m indicates a number of paritycheck bit in one codeword. Accordingly, a difference obtained fromsubtracting the positive integer m by the positive integer n (n-m)indicates a number of information bit (or a message bit) in onecodeword.

FIG. 6 is a schematic diagram illustrating a parity check matrixaccording to an exemplary embodiment.

Referring to FIG. 6, a parity check matrix 600 has a dimension being3-by-8. Generally, the positive integers m and n are greater than 3 and8. FIG. 6 is merely an example of the invention, and values of thepositive integers m and n are not particularly limited. Each row in theparity check matrix 600 also represents a constraint. Take first row asan example, if one codeword is a valid codeword, a bit “0” can beobtained after performing a modulo-2 addition to first, second, fifth,sixth, and eighth bits in the codeword. Persons skilled in the artshould be able to understand how to encode by using the parity checkmatrix 600, thus related description is omitted hereinafter.

When the memory control circuit unit 104 (or the memory managementcircuit 202) writes a plurality of bits to the rewritable non-volatilememory module 106, the error checking and correcting circuit 256correspondingly generates m parity check bits for each (n-m) of the bitsto be written. Then, the memory control circuit unit 104 (or the memorymanagement circuit 202) writes the n bits to the rewritable non-volatilememory module 106. The rewritable non-volatile memory module 106 storesone or more bits by changing a threshold voltage of one memory cell.

FIG. 7 is a schematic diagram illustrating an example of reading therewritable non-volatile memory module according to a hard decisionvoltage according to an exemplary embodiment.

Referring to FIG. 7, a horizontal axis represents the threshold voltageof the memory, and a vertical axis represents a number of the memorycells. For instance, FIG. 7 illustrates the threshold voltage of eachmemory cell on one specific word line. It is assumed that, when thethreshold voltage of the specific memory cell falls in a distribution710, the bit stored in the memory cell is the bit “1”. Otherwise, whenthe threshold voltage of the specific memory cell falls in adistribution 720, the bit stored in the memory cell is the bit “0”. Itis worth mentioning that, the present exemplary embodiment takes aSLC-type flash memory module for example, thus there are two possibledistributions for the threshold voltages. However, in other exemplaryembodiments, the threshold voltages may include four, eight or anynumber of possible distributions, and a hard decision voltage 702 mayfall between any two of the distributions. In addition, the bitrepresented by each distribution is not particularly limited in theinvention.

In the present exemplary embodiment, when it comes to read the data fromthe rewritable non-volatile memory module 106, the memory controlcircuit unit 104 (or the memory management circuit 202) sends a readcommand sequence to the rewritable non-volatile memory module 106. Theread command sequence includes one or more commands or program codes,and is configured to instruct for reading the physical programming unitcomposed of a plurality of memory cells according to the hard decisionvoltage 702 to obtain a plurality of bits. In case the threshold voltageof a specific memory cell is less than the hard decision voltage 702,that specific memory cell is then turned on, and the bit “1” is read bythe memory control circuit unit 104 (or the memory management circuit202). Otherwise, in case the threshold voltage of a specific memory cellis greater than the hard decision voltage 702, that specific memory cellis not turned on, and the bit “0” is read by the memory control circuitunit 104 (or the memory management circuit 202). It should be notedthat, a distribution 710 and a distribution 720 include an overlapregion 730. The overlap region 730 represents that some of the memorycells are supposed to be stored with the bit “1” (which belongs to thedistribution 710) yet having the threshold voltages thereof beinggreater than the hard decision voltage 702; or, some of the memory cellsare supposed to be stored with the bit “0” (which belongs to thedistribution 720) yet having the threshold voltages thereof being lessthan the hard decision voltage 702. In other words, a part of bits amongall the bits being read may have errors. For illustrative convenience,the bits being read according to the hard decision voltage arecollectively referred to as hard bits hereinafter.

In the present exemplary embodiment, each of the hard bits correspondsto one channel information, which is configured to instruct for readingout whether the threshold voltage of the memory cell of the hard bitfall within a stable region or an unstable region. More specifically, ifthe channel information of one hard bit indicates that the thresholdvoltage of the corresponding memory cell falls within the stable region,it indicates that the hard bit has higher probability of being correct.Otherwise, if the channel information of one hard bit indicates that thethreshold voltage of the corresponding memory cell falls within theunstable region, it indicates that the hard bit has higher probabilityof being incorrect. For example, in the exemplary embodiment of FIG. 7,it is assumed that soft decision voltages 704 and 706 are used to cutthe distribution 710 and the distribution 720, such that thedistribution 710 and the distribution 720 may be divided into anunstable region 740, a stable region 750 and a stable region 760. Theunstable region 740 is a region between the soft decision voltages 704and 706; the stable region 750 is a region on the left of the softdecision voltage 704; and the stable region 760 is a region of the rightof the soft decision voltage 706. The unstable region 740 includes atleast a part of the overlap region 730. Generally speaking, if thethreshold voltage of one memory cell falls within the unstable region740, the hard bit read from such memory cell has higher probability ofbeing incorrect. Nonetheless, if the threshold voltage of one memorycell falls within the stable regions 750 or 760, the hard bit read fromsuch memory cell has higher probability of being correct. In thisexemplary embodiment, the soft decision voltages 704 and 706 are decidedaccording to the hard decision voltage 702. Therein, the soft decisionvoltage 704 is less than the hard decision voltage 702, and the softdecision voltage 706 is greater than the hard decision voltage 702. Inan exemplary embodiment, one hard decision voltage may be correspondingto a combination of multiple soft decision voltages, and eachcombination of the soft decision voltages may be recited in a look-uptable for query and use. For example, the soft decision voltages 704 and706 in one combination of the soft decision voltages may be greater thanthe soft decision voltages 704 and 706 in another combination of thesoft decision voltages. In another exemplary embodiment, the softdecision voltages being used each time may be randomly decided accordingto the hard decision voltage 702 within a preset voltage range.

In the present exemplary embodiment, the channel information of one hardbit includes one or more soft bits. The memory control circuit unit 104(or the memory management circuit 202) is capable of reading the memorycells according to the soft decision voltage to obtain the soft bits. Inthe present exemplary embodiment, the command for reading the soft bitsis included in the read command sequence for reading the hard bits.Therefore, whenever one hard bit is read from one memory cell, onecorresponding soft bit is also obtained. Alternatively, in anotherexemplary embodiment, the command for reading the soft bits areimplemented in a read command sequence dedicated for reading the softbits, and such read command sequence may be sent at any time point. Forexample, such read command sequence may only be sent to the rewritablenon-volatile memory module 106 if it is determined that an error occursin one or more hard bits, so as to obtain the corresponding soft bit,but the invention is not limited thereto.

FIG. 8 is a schematic diagram illustrating that the hard bit and thesoft bit corresponding to the distributions of threshold voltages of theSLC-type flash memory module according to an exemplary embodiment.

Referring to FIG. 8, take the SLC-type flash memory module for example,after one read command sequence is issued to the rewritable non-volatilememory module 106, as in correspondence to each of the memory cellsbeing read, the rewritable non-volatile memory module 106 may return onehard bit and the corresponding soft bit. Particularly, a transmissionorder and a transmission time for the hard bit and the soft bit are notparticularly limited in the invention. For example, in an exemplaryembodiment, the rewritable non-volatile memory module 106 may transmitone corresponding soft bit each time one hard bit is sent.Alternatively, in another exemplary embodiment, the rewritablenon-volatile memory module 106 may transmit at least parts of the hardbits first before transmitting at least parts of the corresponding softbits and so on, which is not particularly limited in the invention.

As shown in FIG. 8, in this exemplary embodiment, if the hard bit is “1”and the corresponding soft bit is “0”, it indicates that thecorresponding memory cell may be stored with the bit “1”, and thethreshold voltage of such memory cell falls within the stable region750. If the hard bit being received is “‘1” and the corresponding softbit is “1”, it indicates that the corresponding memory cell may bestored with the bit “1”, and the threshold voltage of such memory cellfalls within the unstable region 740. If the hard bit being received is“‘0” and the corresponding soft bit is “1”, it indicates that thecorresponding memory cell may be stored with the bit “0”, and thethreshold voltage of such memory cell falls within the unstable region740. If the hard bit being received is “0” and the corresponding softbit is “0”, it indicates that the corresponding memory cell may bestored with the bit “0”, and the threshold voltage of such memory cellfalls within the stable region 760. Moreover, in the present exemplaryembodiment, the soft bit is obtained by performing an exclusive OR (XOR)operation for bit values respectively read according to the softdecision voltage 704 and the soft decision voltage 706. However, inanother exemplary embodiment, the soft bit may also be obtained throughother logic operations, which are not particularly limited in theinvention.

FIG. 9 is a schematic diagram illustrating distributions of thresholdvoltages of the MLC-type flash memory module according to an exemplaryembodiment.

Referring to FIG. 9, take a MLC-type flash memory for example, in whicheach of the memory cells has four storage statuses depending on thedifferent threshold voltages, and the storage statuses represent bits“11”, “10”, “00” and “01”, respectively. For example, if the thresholdvoltage of a specific memory cell falls within a distribution 910, thebit stored in that specific memory cell is the bit “11”. If thethreshold voltage of a specific memory cell falls within a distribution920, the bit stored in that specific memory cell is the bit “10”. If thethreshold voltage of a specific memory cell falls within a distribution930, the bit stored in that specific memory cell is the bit “00”. If thethreshold voltage of a specific memory cell falls within a distribution940, the bit stored in that specific memory cell is the bit “01”.

In this exemplary embodiment, each of the memory cells may store twobits. In other words, each of the storage statuses includes a leastsignificant bit (LSB) and a most significant bit (MSB), and the LSB andthe MSB are obtained according to the different hard decision voltagesapplied to the memory cell. In the present exemplary embodiment, amongin the storage statuses (i.e., “11”, “10”, “00” and “01”), a first bitcounted from the left is the LSB, and a second bit counted from the leftis the MSB. In another exemplary embodiment, the storage statusescorresponding to the threshold voltages may also have an arrangement of“11”, “10”, “01” and “00” that is arranged according to the thresholdvoltage from small to large, or other arrangements. In addition, inanother exemplary embodiment, it can also be defined that the first bitcounted from the left is the MSB, and the second bit counted from theleft is the LSB. It should be noted that, in overlap regions 950 to 970,the threshold voltages of some memory cells are overlapping to havehigher probability of having errors in the LSB and the MSB being read.

FIG. 10 and FIG. 11 are schematic diagrams illustrating that the hardbit and the soft bit corresponding to the distributions of thresholdvoltages of the MLC-type flash memory module according to an exemplaryembodiment.

Referring to FIG. 10, take the MLC-type flash memory module for example,when it comes to read the LSB of a specific memory cell, according to ahard decision voltage 1002, a soft decision voltage 1004 and a softdecision voltage 1006 being applied, the distributions 910 to 940 aredivided into an unstable region 1010, a stable region 1020 and a stableregion 1030, and one hard bit and the corresponding soft bit will beobtained. Therein, the hard bit is the LSB of that specific memory cell,and the soft bit indicates, corresponding to the hard bit, the thresholdvoltage of that specific memory cell may fall within the stable regionor the unstable region. As shown in FIG. 10, if the hard bit is “1” andthe corresponding soft bit is “0”, it indicates that the LSB of thecorresponding memory cell may be “1”, and the threshold voltage of suchmemory cell falls within the stable region 1020.

Referring to FIG. 11, when it comes to read the MSB of a specific memorycell, according to a hard decision voltage 1102, a hard decision voltage1104, a soft decision voltage 1106, a soft decision voltage 1108, a softdecision voltage 1110 and a soft decision voltage 1112 being applied,the distributions 910, 920, 930 and 940 are divided into an unstableregion 1120, an unstable region 1130, a stable region 1040, a stableregion 1150 and a stable region 1160, and one hard bit and thecorresponding soft bit will be obtained. Therein, the hard bit is theMSB of that specific memory cell, and the soft bit indicates,corresponding to the hard bit, the threshold voltage of that specificmemory cell may fall within the stable region or the unstable region. Asshown in FIG. 11, if the hard bit is “0” and the corresponding soft bitis “1”, it indicates that the MSB of the corresponding memory cell maybe “0”, and the threshold voltage of such memory cell falls within theunstable region 1120 or the unstable region 1130.

FIG. 12 to FIG. 14 are schematic diagrams illustrating that the hard bitand the soft bit corresponding to the distributions of thresholdvoltages of the TLC-type flash memory module according to an exemplaryembodiment.

Referring to FIG. 12, take a TLC-type flash memory for example, in whichthe memory cells have eight storage statuses (i.e., “111”, “110”, “100”,“101”, “001”, “000”, “010” and “011”) depending on the differentthreshold voltages, and the statuses are corresponding to distributions2210, 2220, 2230, 2240, 2250, 2260, 2270 and 2280, respectively. Each ofthe storage statues includes three bits including: a first bit countedfrom the left being the least significant bit (LSB), a second bitcounted from the left being a center significant bit (CSB), and a thirdbit counted from the left being the most significant bit (MSB). Itshould be noted that, an arranging sequence of the eight storagestatuses may be decided based on designs of manufacturers without beinglimited by the arranging sequence of this embodiment.

When it comes to read the LSB of a specific memory cell, according to ahard decision voltage 1202, a hard decision voltage 1204, a softdecision voltage 1206, a soft decision voltage 1208, a soft decisionvoltage 1210 and a soft decision voltage 1212 being applied, thedistributions 2210, 2220, 2230, 2240, 2250, 2260, 2270 and 2280 aredivided into an unstable region 1220, an unstable region 1230, a stableregion 1240, a stable region 1250 and a stable region 1260, and one hardbit and the corresponding soft bit will be obtained. Therein, the hardbit is the LSB of that specific memory cell, and the soft bit indicates,corresponding to the hard bit, the threshold voltage of that specificmemory cell may fall within the stable region or the unstable region. Asshown in FIG. 12, if the hard bit is “1” and the corresponding soft bitis “1”, it indicates that the LSB of the corresponding memory cell maybe “1”, and the threshold voltage of such memory cell falls within theunstable region 1220 or the unstable region 1230.

Referring to FIG. 13, when it comes to read the CSB of a specific memorycell, according to a hard decision voltage 1302, a hard decision voltage1304, a hard decision voltage 1306, a soft decision voltage 1308, a softdecision voltage 1310, a soft decision voltage 1312, a soft decisionvoltage 1314, a soft decision voltage 1316 and a soft decision voltage1318 being applied, the distributions 2210, 2220, 2230, 2240, 2250,2260, 2270 and 2280 are divided into an unstable region 1320, anunstable region 1330, an unstable region 1340, a stable region 1350, astable region 1360, a stable region 1370 and a stable region 1380, andone hard bit and the corresponding soft bit will be obtained. Therein,the hard bit is the CSB of that specific memory cell, and the soft bitindicates, corresponding to the hard bit, the threshold voltage of thatspecific memory cell may fall within the stable region or the unstableregion. As shown in FIG. 13, if the hard bit is “0” and thecorresponding soft bit is “1”, it indicates that the CSB of thecorresponding memory cell may be “0”, and the threshold voltage of suchmemory cell falls within the unstable region 1320, the unstable region1330 or the unstable region 1340.

Referring to FIG. 14, when it comes to read the MSB of a specific memorycell, according to a hard decision voltage 1402, a hard decision voltage1404, a soft decision voltage 1406, a soft decision voltage 1408, a softdecision voltage 1410 and a soft decision voltage 1412 being applied,the distributions 2210, 2220, 2230, 2240, 2250, 2260, 2270 and 2280 aredivided into an unstable region 1420, an unstable region 1430, a stableregion 1440, a stable region 1450 and a stable region 1460, and one hardbit and the corresponding soft bit will be obtained. Therein, the hardbit is the MSB of that specific memory cell, and the soft bit indicates,corresponding to the hard bit, the threshold voltage of that specificmemory cell may fall within the stable region or the unstable region. Asshown in FIG. 14, if the hard bit is “1” and the corresponding soft bitis “0”, it indicates that the MSB of the corresponding memory cell maybe “1”, and the threshold voltage of such memory cell falls within thestable region 1440. It is worth mentioning that, in any one among theexemplary embodiments of FIG. 7 to FIG. 14, values and amounts of thesoft decision voltages and amounts of the soft bits may be adaptivelyincreased or decreased, which are not particularly limited in theinvention. In other words, according to the soft bits, the memorycontrol circuit unit 104 (or the memory management circuit 202) may beroughly informed of whether the threshold voltage of the correspondingmemory cell falls within the stable region or the unstable region.Further, with reference by the hard bit, the memory control circuit unit104 (or the memory management circuit 202) may be more specificallyinformed of whether the threshold voltage of the corresponding memorycells falls with which one (or more) of the stable regions or which one(or more) of the unstable regions. For example, in the exemplaryembodiment of FIG. 14, when the soft bit is 0, it indicates that thethreshold voltage of the corresponding memory cell may fall within oneamong the stable region 1440, the stable region 1450 and the stableregion 1460. Further, based on the hard bit being “1”, it indicates thatthe threshold voltage of the corresponding memory is more likely to fallwithin the stable region 1440 instead of the stable region 1450 and thestable region 1460. In other words, the memory control circuit unit 104(or the memory management circuit 202) may determine whether thresholdvoltage of the memory cell falls within the stable region or theunstable region according to only the soft bit, or according to both thehard bit and the corresponding soft bit, which is not particularlylimited in the invention.

After the memory control circuit unit 104 (or the memory managementcircuit 202) reads the hard bits from the rewritable non-volatile memorymodule 106 according to the hard decision voltage (e.g., the harddecision voltage 702 of FIG. 7), the hard bits are divided into one ormore codewords having a length being n. The error checking andcorrecting circuit 256 decodes each of the codewords. More specifically,the error checking and correcting circuit 256 first performs a paritycheck of the low density parity check algorithm for the hard bits toobtain a plurality of syndromes. For instance, the error checking andcorrecting circuit 256 may perform a modulo-2 matrix multiplication tothe parity check matrix and one codeword, and the modulo-2 matrixmultiplication may be represented by the following equation (1).

[H][V]=[S]  (1)

H is the parity check matrix. V is one codeword with the dimension beingn-by-1. S is a syndrome vector including the syndromes, and a dimensionof the syndrome vector is m-by-1. The error checking and correctingcircuit 256 may determine whether the hard bit in the codeword V has theerror according to the syndromes. More specifically, in case each of thesyndromes in the syndrome vector S is the bit “0”, this indicates thatthe codeword V may not have the error. In case one or more of thesyndromes in the syndrome vector S is the bit “1”, this indicates thatthe codeword V has at least one error.

FIG. 15 is a schematic diagram illustrating a matrix multiplicationaccording to an exemplary embodiment.

Referring to FIG. 15, a result of multiplying the parity check matrix600 by a codeword 1510 is a syndrome vector 1520. Each of the hard bitsin the codeword 1510 is corresponding to at least one syndrome in thesyndrome vector 1520. For instance, a first hard bit V₀ (which iscorresponding to first row of the parity check matrix 600) in thecodeword 1510 is corresponding to a syndrome S₀; and a hard bit V₁(which is corresponding to second row of the parity check matrix 600) iscorresponding to a syndrome S₀ and a syndrome S₁. If the error occurs inthe hard bit V₀, the syndrome S₀ may be the bit “1”. If the error occursin the bit V₁, the syndromes S₀ and S₁ may be the bit “1”. In otherwords, in case an element at an i^(th) column and a j^(th) row in theparity check matrix 600 is “1”, a j^(th) bit in the codeword 1510 is atleast corresponding to an i^(th) syndrome in the syndrome vector 1520,and i and j are positive integers.

If the hard bits in the codeword 1510 do not have error, the errorchecking and correcting circuit 256 may output the hard bits in thecodeword 1510. If the hard bits in the codeword 1510 have the error, theerror checking and correcting circuit 256 may perform an iterationcalculation to update the codeword 1510 according to the channelinformation of each of the hard bits in the codeword 1510 and syndromeweight information corresponding to each of the hard bits. Morespecifically, the error checking and correcting circuit 256 obtains thesyndrome weight information corresponding to each of the hard bitsaccording to the syndromes corresponding to each of the hard bits. Forinstance, the error checking and correcting circuit 256 may add thesyndromes corresponding to the same bits together, so as to obtain thesyndrome weight information corresponding to such hard bit. As shown inFIG. 15, the syndrome weight information corresponding to the hard bitV₀ is equal to the syndrome S₀; the syndrome weight informationcorresponding to the hard bit V₁ is equal to a sum of the syndrome S₀and syndrome S₁, the rest may be deduced by analogy. It should be notedthat, the addition for the syndromes S₀ to S₂ is a normal additioninstead of the modulo-2 addition. However, in another exemplaryembodiment, the error checking and correcting circuit 256 may alsomultiply each of the syndromes by a weight, and accumulate a result ofmultiplying the syndromes by the weight to obtain the syndrome weightinformation. For instance, the syndrome weight information correspondingto the hard bit V₁ is equal to W₀S₀+W₁S₁, and the weights W₀ and W₁ arereal numbers. The error checking and correcting circuit 256 may decidethe weights according to amount of the hard bits corresponding to thesyndromes. For instance, the syndrome S₀ is corresponding to 5 hardbits, and the syndrome S₁ is corresponding to 3 hard bits. Accordingly,the error checking and correcting circuit 256 may set the weight W₀ asto be less than (or greater than) the weight W₁. Methods for setting theweight of each of the syndromes are not particularly limited in theinvention. In another exemplary embodiment, the error checking andcorrecting circuit 256 may also use at least one of the syndromes S₀ toS₂ as an input of a function, and uses the output of the function as thesyndrome weight information. The function can be a linear function, apolynomial function, an exponential function or other nonlinearfunctions, and the invention is not limited to the above.

After the syndrome weight information corresponding to each of the hardbits is obtained, the error checking and correcting circuit 256 maydetermine whether the syndrome weight information corresponding to eachof the hard bits in the codeword 1510 matches a weight condition. Forexample, the error checking and correcting circuit 256 may compare thesyndrome weight information corresponding to each of the hard bits, andconsider that a N number of the syndrome weight information with thelargest value are the syndrome weight information matching the weightcondition. Therein, N is a positive integer. For instance, assuming thatN=4, the syndrome weight information corresponding to the hard bits V₀to V₇ obtained by the error checking and correcting circuit 256 arerespectively “1”, “2”, “1”, “1”, “2”, “2”, “1” and “2”, and the syndromeweight information corresponding to the hard bits V₁, V₄, V₅ and V₇ arethe four with the largest value, thus the error checking and correctingcircuit 256 may consider that the syndrome weight informationcorresponding to the hard bits V₁, V₄, V₅ and V₇ are the syndrome weightinformation matching the weight condition. Nevertheless, if N is lessthan 4, the error checking and correcting circuit 256 may select one tothree from among the hard bits V₁, V₄, V₅ and V₇ randomly or inaccordance with other rules to serve as the hard bit(s) corresponding tothe syndrome weight information matching the weight condition. Next,assuming that the syndrome weight information (also known as a firstsyndrome weight information) corresponding to the hard bit V₁ matchesthe weight condition, the error checking and correcting circuit 256 mayfurther determine whether the channel information of the hard bit V₁matches a channel condition. For example, the error checking andcorrecting circuit 256 may determine whether one or more soft bits (alsoknown as a first soft bit) corresponding to the hard bit V₁ matches afirst status. If the first soft bit matches the first status, the errorchecking and correcting circuit 256 may determine that the channelinformation of the hard bit V₁ matches the channel condition. Otherwise,if the first soft bit does not match the first status, the errorchecking and correcting circuit 256 may determine that the channelinformation of the hard bit V₁ does not match the channel condition.

For example, assuming that the first soft bit includes only one bit, theerror checking and correcting circuit 256 may then determine whether thefirst soft bit is a first value. As continuation to the exemplaryembodiments of FIG. 7 to FIG. 14, the first value is, for example, “1”,and is configured to indicate that the threshold voltage of the memorycell corresponding to the hard bit V₁ falls within the unstable region.In case the first soft bit is the first value, the error checking andcorrecting circuit 256 may then determine that the first soft bitmatches the first status. Otherwise, in case the first soft bit is notthe first value, the error checking and correcting circuit 256 may thendetermine that the first soft bit does not match the first status. Forexample, as continuation to the exemplary embodiments of FIG. 7 to FIG.14, if the value of the first soft bit corresponding to the hard bit V₁is “0”, it indicates that the threshold voltage of the memory cellcorresponding to the hard bit V₁ falls within the stable region, and theerror checking and correcting circuit 256 determines that the first softbit does not match the first status. In addition, in case the first softbit includes a plurality of bits, the first status may be configured tolimit a status of each bit of the first soft bit. For example, if thefirst soft bit includes three bits, the first status may be “111”, butnot limited thereto.

If the channel information of the hard bit V₁ matches the channelcondition, the error checking and correcting circuit 256 updates thehard bit V₁. For example, the hard bit V₁ may be updated from “0” into“1”, or the hard bit V₁ may be updated from “1” into “0”. Otherwise, ifthe channel information of the hard bit V₁ does not match the channelcondition, the error checking and correcting circuit 256 does not updatethe hard bit V₁. In an exemplary embodiment, the operation of updatingthe hard bit may also be referred to as a bit flipping.

Moreover, in an exemplary embodiment, if the hard bit in the codeword1510 has the error, the error checking and correcting circuit 256 mayalso decide which one of the hard bits in the codeword 1510 is to beupdated simply according to the channel information of each of the hardbits in the codeword 1510. For example, the error checking andcorrecting circuit 256 may check whether the channel information of eachof the hard bits in the codeword 1510 matches said channel condition,and only update the hard bits having the channel information matchingthe channel condition. Accordingly, computations may be reduced toincrease a speed of each iteration calculation.

After the hard bits required to be updated in the codeword 1510 are allupdated, the error checking and correcting circuit 256 may perform theparity checking procedure for the hard bits in the updated codeword 1510to obtain the syndromes again, and determine whether the hard bits inthe updated codeword 1510 still has the at least one error according tothe syndromes being obtained again. If the hard bits in the updatedcodeword 1510 still have the error, the error checking and correctingcircuit 256 may perform the aforesaid operation of updating the hardbits according to the channel information of each of the hard bits inthe codeword 1510 and the syndrome weight information corresponding toeach of the hard bits again. Otherwise, if the hard bits in the codeword1510 do not have the error, the error checking and correcting circuit256 may output the updated hard bits in the codeword 1510.

In an exemplary embodiment, if the error checking and correcting circuit256 determines that the codeword 1510 has the error, the error checkingand correcting circuit 256 may count a number of iterations (e.g., byadding one to the number of iterations), and determine whether thenumber of iterations reaches a suspend number. Herein, the suspendnumber may be 30 times, or more or less for example. If the number ofiterations being counted reaches the suspend number, the error checkingand correcting circuit 256 may determine that decoding fails, and stopdecoding. If the number of iterations being counted does not reach thesuspend number, the error checking and correcting circuit 256 mayperform the aforesaid operation of updating the hard bits according tothe channel information of each of the hard bits in the codeword 1510and the syndrome weight information corresponding to each of the hardbits.

In an exemplary embodiment, the error checking and correcting circuit256 may also determine whether the number of iterations being countedreaches a preset number. Herein, the preset number is less than thesuspend number, and may be 20 times, or more or less, for example. Ifthe number of iterations being counted does not reach the preset number,the error checking and correcting circuit 256 may set the channelcondition to a first channel condition. If the number of iterationsbeing counted reaches the preset number, the error checking andcorrecting circuit 256 may set the channel condition to a second channelcondition. Therein, the first channel condition is different from thesecond channel condition. For instance, in the exemplary embodiment ofFIG. 8, assuming that the soft bit corresponding to a specific hard bitis “0” in the first 20 times of the iteration calculations, it indicatesthat specific hard bit is not updated. After 20 times of the iterationcalculations, if the error checking and correcting circuit 256 stillcannot obtain a correct codeword, the channel condition may be relaxedby the error checking and correcting circuit 256, so that the specifichard bit may be updated to increase probability of obtaining the correctcodeword.

FIG. 16 is a schematic diagram illustrating that the hard bit and thesoft bit corresponding to the distributions of threshold voltages of theSLC-type flash memory module according to an exemplary embodiment.

Referring to FIG. 16, in the present exemplary embodiment, the channelinformation of each of the hard bits includes a plurality of soft bits.The soft bits have a plurality of statuses. Based on different statusesof the soft bits, the threshold voltage of the memory cell correspondingto the hard bit may be more specifically classified into one of aplurality of hierarchies. Referring to FIG. 16, when it comes to read aspecific memory cell, according to a hard decision voltage 1602, a softdecision voltage 1604, a soft decision voltage 1606, a soft decisionvoltage 1608, a soft decision voltage 1610, a soft decision voltage 1612and a soft decision voltage 1614 being applied, the distributions 710and 720 are divided into an unstable region 1620, a stable region 1630,a stable region 1640, a stable region 1650, a stable region 1660, astable region 1670 and a stable region 1680, and one hard bit and thecorresponding soft bit (1) to the soft bit (3) will be obtained.Therein, the soft bit (1) is obtain by performing the exclusive ORoperation for the bit values respectively read according to the softdecision voltage 1604 and the soft decision voltage 1606; the soft bit(2) is obtain by performing the exclusive OR operation for the bitvalues respectively read according to the soft decision voltage 1608 andthe soft decision voltage 1610; and the soft bit (3) is obtain byperforming the exclusive OR operation for the bit values respectivelyread according to the soft decision voltage 1612 and the soft decisionvoltage 1614. For instance, if the status of the soft bit (1) to thesoft bit (3) is “111”, it indicates that the threshold voltage of thememory cell corresponding to the hard bit falls within the unstableregion 1620; if the hard bit is “0” and the status of the soft bit (1)to the soft bit (3) is “011”, it indicates that the threshold voltage ofthe memory cell corresponding to the hard bit falls within the stableregion 1640; and the rest may be deduced by analogy. In other words, ifthe status of the soft bit (1) to the soft bit (3) is “000”, itindicates that the probability for the error to occur on such hard bitis the smallest. If the status of the soft bit (1) to the soft bit (3)is “001” or “011”, it indicates that the probability for the error tooccur on such hard bit gradually increases. If the status of the softbit (1) to the soft bit (3) is “111”, it indicates that the probabilityfor the error to occur on such hard bit is the highest. However, inanother exemplary embodiment, if the logic operation (e.g., theexclusive OR operation) is not performed for the soft bits being read,or the amount of the logic operations is increased/decreased, the amountof the soft bits included in the channel information may be more orless.

In the present exemplary embodiment, assuming that throughout theiteration calculations of a first to a P^(th) times, the preset channelcondition is: the status of the soft bit (1) to the soft bit (3) being“111”. Throughout the iteration calculations of the first to the P^(th)times, even if the syndrome weight information corresponding to aspecific hard bit matches the weight condition, as long as the status ofthe soft bit (1) to the soft bit (3) corresponding to that specific hardbit is not “111”, the specific hard bit will not be updated. After theiteration calculation of the p^(th) time is completed, the channelcondition is changed to: the status of the soft bit (1) to the soft bit(3) being “011”. Accordingly, after the iteration calculation of theP^(th) time is completed, if the syndrome weight information of aspecific hard bit matches the weight condition, and the soft bit (1) tothe soft bit (3) corresponding to that specific hard bit matches thechannel condition being changed, that specific hard bit may then beupdated to increase probability of obtaining the correcting codeword. Inaddition, throughout the iteration calculations of a (P+1)^(th) to aQ^(th) times, the channel condition may also be set to: the status ofthe soft bit (1) to the soft bit (3) being “001”; and, throughout theiteration calculations of a (Q+1)^(th) to a R^(th) times, the channelcondition may also be set to: no specific conditions, or the channelcondition only being applied with minor changes. Therein, P, Q and R arepositive integers, and R>Q>P. Moreover, in an exemplary embodiment, thechannel condition may also be set to: the status of the soft bit (1) andthe soft bit (3) always being “111”, or there is always no condition,which are not changed with increases in the number of iterations.

It is worth mentioning that, the channel information of the hard bit isnot necessarily to be obtained by reading the memory cells according tothe soft decision voltage. In an exemplary embodiment, the memorycontrol circuit unit 104 (or the memory management circuit 202) may alsoread a read result of a specific memory cell according to different harddecision voltages, so as to be informed of whether the threshold voltageof that specific memory cell falls within the stable region or theunstable region, and thereby decide the channel information of the hardbit read from that specific memory cell.

More specifically, take FIG. 15 as an example, in an exemplaryembodiment, assuming that the memory control circuit unit 104 (or thememory management circuit 202) is initially not informed of whether thethreshold voltage of each of the memory cells composing the physicalprogramming unit falls within the stable region or the unstable region.Therefore, after the hard bits in the codeword 1510 are obtained, thememory control circuit unit 104 (or the memory management circuit 202)may consider that the channel information of each of the hard bits incodeword 1510 matches the channel condition, and update the hard bitsthat needs to be updated in the codeword 1510 according to the syndromeweight information corresponding to each of the hard bits. Then, ifdecoding fails, the memory control circuit unit 104 (or the memorymanagement circuit 202) may obtain at least one new hard decisionvoltage by querying a look-up table, or slightly increasing ordecreasing the hard decision voltage (i.e., the old hard decisionvoltage), and then send a new read command sequence to the rewritablenon-volatile memory module 106. The new read command sequence includesone or more commands or program codes, and is configured to instruct forreading the physical programming unit composed of a plurality of memorycells according to the new hard decision voltage to obtain a pluralityof new hard bits. Thereafter, the memory control circuit unit 104 (orthe memory management circuit 202) may compare whether the hard bitsread from a specific memory cell according to the old hard decisionvoltage is identical to the new hard bits read from that specific memorycell according to the new hard decision voltage. If they are identical,it indicates that the threshold voltage of the memory cell falls withinthe stable region; and if they are not identical, it indicates that thethreshold voltage of the memory cell falls within the unstable region.The memory control circuit unit 104 (or the memory management circuit202) may decide the channel information of the new hard bits based onwhether the threshold voltage of the memory cell falls within the stableregion or the unstable region. For example, if it is determined that thethreshold voltage falls within the stable region, the memory controlcircuit unit 104 (or the memory management circuit 202) may set the softbit corresponding to the new hard bit to “0”; and if it is determinedthat the threshold voltage falls within the unstable region, the memorycontrol circuit unit 104 (or the memory management circuit 202) may setthe soft bit corresponding to the new hard bit to “1”, but the inventionis not limited thereto. Alternatively, after the memory cell is read byusing a specific amount of hard decision voltages, the memory controlcircuit unit 104 (or the memory management circuit 202) may alsodetermine that the threshold voltage of the memory cell falls within theunstable region if a number of changes (which includes changing from “0”to “1” and changing from “1” to “0”) of the hard bits being read exceedsa specific number of times, or else determine that threshold voltagefalls within the stable region. In addition, this method of deciding thechannel information according to the number of changes of the hard bitsmay be used together with the soft bits obtained according to the softdecision voltage, which is not particularly limited in the invention.

It is worth mentioning that, in each of the foregoing exemplaryembodiments, the stable region may also be named as a high reliableregion or names with similar meaning, and the unstable region may alsobe named as a low reliable region or names with similar meaning, or theycan be named in a hierarchical way. For example, in the exemplaryembodiment of FIG. 16, the region 1620 may be named as a lowest stable(reliable) region; the region 1630 and the region 1640 may be named assecond lowest stable (reliable) regions; the region 1650 and the region1660 may be named as second highest stable (reliable) regions; and theregion 1670 and the region 1680 may be named as highest stable(reliable) regions, but the invention is not limited thereto.

FIG. 17 is a flowchart illustrating a decoding method according to anexemplary embodiment.

Referring to FIG. 17, in step S1702, the memory cells are read accordingto at least one hard decision voltage to obtain at least one hard bit.

In step S1704, a parity checking procedure is performed for the hard bitto obtain a plurality of syndromes, wherein each of the hard bit iscorresponding to at least one of the syndromes.

In step S1706, whether the hard bit has at least one error is determinedaccording to the syndromes.

If the hard bit has the error, the hard bit is updated according tochannel information of the hard bit and syndrome weight informationcorresponding to the hard bit in step S1708. Thereafter, step S1704 isrepeated.

If the hard bit does not have the error, the hard bit is outputted instep S1710.

FIG. 18 is a flowchart illustrating a decoding method according toanother exemplary embodiment.

Referring to FIG. 18, in step S1802, the memory cells are read accordingto at least one hard decision voltage to obtain at least one hard bit.

In step S1804, a parity checking procedure is performed for the hard bitto obtain a plurality of syndromes, wherein each of the hard bit iscorresponding to at least one of the syndromes.

In step S1806, whether the hard bit has at least one error is determinedaccording to the syndromes.

If the hard bit does not have the error, the hard bit is outputted instep S1808.

If the hard bit has the error, a number of iterations is counted in stepS1810, and whether the number of iterations reaches a suspend number isdetermined in step S1812.

If the number of iterations reaches the suspend number, it is determinedthat decoding fails in step S1814.

If the number of iterations does not reach the suspend number, whetherthe number of iterations reaches a preset number is determined in stepS1816. Herein, the preset number is less than the suspend number.

If the number of iterations does not reach the preset number, thechannel condition is set to a first channel condition in step S1818. Ifthe channel condition at the time is already the first channelcondition, step S1818 may be omitted.

If the number of iterations reaches the preset number, the channelcondition is set to a second channel condition in step S1820.

In step S1822, the hard bit is updated according to channel informationof the hard bit and syndrome weight information corresponding to thehard bit. Thereafter, step S1804 is repeated.

In an exemplary embodiment, whether a number of times that decodingfails reaches a specific number of times may be determined in stepS1814. If the specific number of times is not yet reached, step S1802 isperformed repeatedly. For example, the memory control circuit unit 104(or the memory management circuit 202) may issue another read commandsequence to the rewritable non-volatile memory module 106, so as to readthe memory cells according to another hard decision voltage which isdifferent from the hard decision voltage being used previously, andobtain the at least one hard bit again. Thereafter, step S1804 and stepS1806 and so on are successively performed. Otherwise, if the number oftimes that decoding fails reaches the specific number of times, thisdecoding procedure is stopped.

Nevertheless, each of steps depicted in FIG. 17 and FIG. 18 have beendescribed in detail as above, thus related description thereof isomitted hereinafter. It should be noted that, the steps depicted in FIG.17 and FIG. 18 may be implemented as a plurality of program codes orcircuits, and the invention is not limited thereto. Moreover, themethods disclosed in FIG. 17 and FIG. 18 may be implemented withreference to above embodiments, or may be implemented separately, andthe invention is not limited thereto.

In summary, in the decoding method, the memory storage device and thememory control circuit unit in an embodiment of the invention, the bitflipping is adopted during the error correction, and the channelinformation related to the threshold voltage of the memory cell is alsoused together to facilitate in ensuring the bits to be flipped, so as toeffectively improve decoding efficiency.

The previously described exemplary embodiments of the present inventionhave the advantages aforementioned, wherein the advantagesaforementioned not required in all versions of the invention.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A decoding method for a rewritable non-volatilememory module comprising a plurality of memory cells, and the decodingmethod comprising: reading the memory cells according to at least onehard decision voltage to obtain at least one hard bit; performing aparity checking procedure for the at least one hard bit to obtain aplurality of syndromes, wherein each of the at least one hard bit iscorresponding to at least one of the syndromes; determining whether theat least one hard bit has at least one error according to the syndromes;if the at least one hard bit has the at least one error, updating the atleast one hard bit according to channel information of the at least onehard bit and syndrome weight information corresponding to the at leastone hard bit; and if the at least one hard bit does not have the atleast one error, outputting the at least one hard bit.
 2. The decodingmethod of claim 1, wherein the at least one hard bit comprises a firsthard bit, if the at least one hard bit has the at least one error, thestep of updating the at least one hard bit according to the channelinformation of the at least one hard bit and the syndrome weightinformation corresponding to the at least one hard bit comprises:determining whether first syndrome weight information corresponding tothe first hard bit among the syndrome weight information matches aweight condition; if the first syndrome weight information matches theweight condition, determining whether the channel information of thefirst hard bit matches a channel condition; if the channel informationof the first hard bit matches the channel condition, updating the firsthard bit; and if the channel information of the first hard bit does notmatch the channel condition, not updating the first hard bit.
 3. Thedecoding method of claim 2, wherein the channel information of the firsthard bit comprises at least one first soft bit, and the step ofdetermining whether the channel information of the first hard bitmatches the channel condition comprises: determining whether the atleast one first soft bit matches a first status; if the at least onefirst soft bit matches the first status, determining that the channelinformation of the first hard bit matches the channel condition; and ifthe at least one first soft bit does not match the first status,determining that the channel information of the first hard bit does notmatch the channel condition.
 4. The decoding method of claim 2, whereinif the at least one hard bit has the at least one error, the decodingmethod further comprises: counting a number of iterations; determiningwhether the number of iterations reaches a preset number; if the numberof iterations does not reach the preset number, setting the channelcondition to a first channel condition; and if the number of iterationsreaches the preset number, setting the channel condition to a secondchannel condition, wherein the first channel condition is different fromthe second channel condition.
 5. The decoding method of claim 1, furthercomprising: reading the memory cells according to at least one softdecision voltage to obtain the channel information of the at least onehard bit.
 6. The decoding method of claim 5, wherein the at least onehard decision voltage comprises a first hard decision voltage, the atleast one soft decision voltage comprises a first soft decision voltageand a second soft decision voltage, the first soft decision voltage isless than the first hard decision voltage, and the second soft decisionvoltage is greater than the first hard decision voltage.
 7. The decodingmethod of claim 1, further comprising: if decoding fails, reading thememory cells according to at least one new hard decision voltage toobtain at least one new hard bit; and deciding the channel informationof the at least one new hard bit according to the at least one hard bitand the at least one new hard bit.
 8. The decoding method of claim 1,wherein if the at least one hard bit has the at least one error, thedecoding method further comprises: counting a number of iterations;determining whether the number of iterations reaches a suspend number;if the number of iterations reaches the suspend number, determining thatdecoding fails; and if the number of iterations does not reach thesuspend number, performing the parity checking procedure for the atleast one updated hard bit to obtain the syndromes again, anddetermining whether the at least one updated hard bit has the at leastone error according to the syndromes being obtained again.
 9. A memorystorage device, comprising: a connection interface unit configured tocouple to a host system; a rewritable non-volatile memory modulecomprising a plurality of memory cells; and a memory control circuitunit coupled to the connection interface unit and the rewritablenon-volatile memory module, wherein the memory control circuit unit isconfigured to read the memory cells according to at least one harddecision voltage to obtain at least one hard bit, the memory controlcircuit unit is further configured to perform a parity checkingprocedure for the at least one hard bit to obtain a plurality ofsyndromes, wherein each of the at least one hard bit is corresponding toat least one of the syndromes, the memory control circuit unit isfurther configured to determine whether the at least one hard bit has atleast one error according to the syndromes, if the at least one hard bithas the at least one error, the memory control circuit unit is furtherconfigured to update the at least one hard bit according to channelinformation of the at least one hard bit and syndrome weight informationcorresponding to the at least one hard bit, and if the at least one hardbit does not have the at least one error, the memory control circuitunit is further configured to output the at least one hard bit.
 10. Thememory storage device of claim 9, wherein the at least one hard bitcomprises a first hard bit, if the at least one hard bit has the atleast one error, the operation of the memory control circuit unit forupdating the at least one hard bit according to the channel informationof the at least one hard bit and the syndrome weight informationcorresponding to the at least one hard bit comprises: determiningwhether first syndrome weight information corresponding to the firsthard bit among the syndrome weight information matches a weightcondition; if the first syndrome weight information matches the weightcondition, determining whether the channel information of the first hardbit matches a channel condition; if the channel information of the firsthard bit matches the channel condition, updating the first hard bit; andif the channel information of the first hard bit does not match thechannel condition, not updating the first hard bit.
 11. The memorystorage device of claim 10, wherein the channel information of the firsthard bit comprises at least one first soft bit, and the operation of thememory control circuit unit for determining whether the channelinformation of the first hard bit matches the channel conditioncomprises: determining whether the at least one first soft bit matches afirst status; if the at least one first soft bit matches the firststatus, determining that the channel information of the first hard bitmatches the channel condition; and if the at least one first soft bitdoes not match the first status, determining that the channelinformation of the first hard bit does not match the channel condition.12. The memory storage device of claim 10, wherein if the at least onehard bit has the at least one error, the memory control circuit unit isfurther configured to count a number of iterations, the memory controlcircuit unit is further configured to determine whether the number ofiterations reaches a preset number, if the number of iterations does notreach the preset number, the memory control circuit unit is furtherconfigured to set the channel condition to a first channel condition,and if the number of iterations reaches the preset number, the memorycontrol circuit unit is further configured to set the channel conditionto a second channel condition, wherein the first channel condition isdifferent from the second channel condition.
 13. The memory storagedevice of claim 9, wherein the memory control circuit unit is furtherconfigured to read the memory cells according to at least one softdecision voltage to obtain the channel information of the at least onehard bit.
 14. The memory storage device of claim 13, wherein the atleast one hard decision voltage comprises a first hard decision voltage,the at least one soft decision voltage comprises a first soft decisionvoltage and a second soft decision voltage, the first soft decisionvoltage is less than the first hard decision voltage, and the secondsoft decision voltage is greater than the first hard decision voltage.15. The memory storage device of claim 9, wherein if decoding fails, thememory control circuit unit is further configured to read the memorycells according to at least one new hard decision voltage to obtain atleast one new hard bit, and the memory control circuit unit is furtherconfigured to decide the channel information of the at least one newhard bit according to the at least one hard bit and the at least one newhard bit.
 16. The memory storage device of claim 9, wherein if the atleast one hard bit has the at least one error, the memory controlcircuit unit is further configured to count a number of iterations, thememory control circuit unit is further configured to determine whetherthe number of iterations reaches a suspend number, if the number ofiterations reaches the suspend number, the memory control circuit unitis further configured to determine that decoding fails, and if thenumber of iterations does not reach the suspend number, the memorycontrol circuit unit is further configured to perform the paritychecking procedure for the at least one updated hard bit to obtain thesyndromes again, and determine whether the at least one updated hard bithas the at least one error according to the syndromes being obtainedagain.
 17. A memory control circuit unit, for a rewritable non-volatilememory module, wherein the rewritable non-volatile memory modulecomprises a plurality of memory cells, and the memory control circuitunit comprises: a host interface configured to couple to a host system;a memory interface configured to couple to the rewritable non-volatilememory module; an error checking and correcting circuit; and a memorymanagement circuit coupled to the host interface, the memory interfaceand the error checking and correcting circuit, wherein the memorymanagement circuit is configured to send a read command sequence,wherein the read command sequence is configured to instruct for readingthe memory cells according to at least one hard decision voltage toobtain at least one hard bit, the error checking and correcting circuitis configured to perform a parity checking procedure for the at leastone hard bit to obtain a plurality of syndromes, wherein each of the atleast one hard bit is corresponding to at least one of the syndromes,the error checking and correcting circuit is further configured todetermine whether the at least one hard bit has at least one erroraccording to the syndromes, if the at least one hard bit has the atleast one error, the error checking and correcting circuit is furtherconfigured to update the at least one hard bit according to channelinformation of the at least one hard bit and syndrome weight informationcorresponding to the at least one hard bit, and if the at least one hardbit does not have the at least one error, the memory management circuitis further configured to output the at least one hard bit.
 18. Thememory control circuit unit of claim 17, wherein the at least one hardbit comprises a first hard bit, if the at least one hard bit has the atleast one error, the operation of the error checking and correctingcircuit for updating the at least one hard bit according to the channelinformation of the at least one hard bit and the syndrome weightinformation corresponding to the at least one hard bit comprises:determining whether first syndrome weight information corresponding tothe first hard bit among the syndrome weight information matches aweight condition; if the first syndrome weight information matches theweight condition, determining whether the channel information of thefirst hard bit matches a channel condition; if the channel informationof the first hard bit matches the channel condition, updating the firsthard bit; and if the channel information of the first hard bit does notmatch the channel condition, not updating the first hard bit.
 19. Thememory control circuit unit of claim 18, wherein the channel informationof the first hard bit comprises at least one first soft bit, and theoperation of the error checking and correcting circuit for determiningwhether the channel information of the first hard bit matches thechannel condition comprises: determining whether the at least one firstsoft bit matches a first status; if the at least one first soft bitmatches the first status, determining that the channel information ofthe first hard bit matches the channel condition; and if the at leastone first soft bit does not match the first status, determining that thechannel information of the first hard bit does not match the channelcondition.
 20. The memory control circuit unit of claim 18, wherein ifthe at least one hard bit has the at least one error, the error checkingand correcting circuit is further configured to count a number ofiterations, the error checking and correcting circuit is furtherconfigured to determine whether the number of iterations reaches apreset number, if the number of iterations does not reach the presetnumber, the error checking and correcting circuit is further configuredto set the channel condition to a first channel condition, and if thenumber of iterations reaches the preset number, the error checking andcorrecting circuit is further configured to set the channel condition toa second channel condition, wherein the first channel condition isdifferent from the second channel condition.
 21. The memory controlcircuit unit of claim 17, wherein the read command sequence is furtherconfigured to instruct for reading the memory cells according to atleast one soft decision voltage to obtain the channel information of theat least one hard bit.
 22. The memory control circuit unit of claim 21,wherein the at least one hard decision voltage comprises a first harddecision voltage, the at least one soft decision voltage comprises afirst soft decision voltage and a second soft decision voltage, thefirst soft decision voltage is less than the first hard decisionvoltage, and the second soft decision voltage is greater than the firsthard decision voltage.
 23. The memory control circuit unit of claim 17,wherein if decoding fails, the memory management circuit is furtherconfigured to send a new read command sequence, wherein the new readcommand sequence is configured to instruct for reading the memory cellsaccording to at least one new hard decision voltage to obtain at leastone new hard bit, and the memory management circuit is furtherconfigured to decide the channel information of the at least one newhard bit according to the at least one hard bit and the at least one newhard bit.
 24. The memory control circuit unit of claim 17, wherein ifthe at least one hard bit has the at least one error, the error checkingand correcting circuit is further configured to count a number ofiterations, the error checking and correcting circuit is furtherconfigured to determine whether the number of iterations reaches asuspend number, if the number of iterations reaches the suspend number,the error checking and correcting circuit is further configured todetermine that decoding fails, and if the number of iterations does notreach the suspend number, the error checking and correcting circuit isfurther configured to perform the parity checking procedure for the atleast one updated hard bit to obtain the syndromes again, and determinewhether the at least one updated hard bit has the at least one erroraccording to the syndromes being obtained again.